The present invention relates to a thin film semiconductor device, and in particular, to a thin film transistor having an inverted stagger structure to be applied to liquid crystal display devices, its fabrication method, and image processor devices.
As thin film transistors (TFTs) applied to liquid crystal display devices, there have been amorphous silicon (a-Si) TFTs having inverted stagger structures. On the other hand, there have been some attempts to improve the carrier mobility in TFTs, such as set forth in the Japanese Patent Publication No.59-15524 (1984) where TFTs having a coplanar structure with an a-Si film being formed under a p-Si (polycrystalline silicon) film were used, or in Japanese Patent Publication No.62-295465 (1987) where an a-Si layer was provided between the source and drain regions, and the channel region (p-Si layer). Further, a laser annealing method for realizing a two-layer structure of a-Si and p-Si by laser annealing part of the a-Si film into p-Si was described in IEEE Trans. Electron Dev.36,2868-2872 (1989).
In these prior arts, however, annealing had been conducted in vacuum or inert gas, with temperatures of the substrates set at room temperatures. By laser annealing in vacuum or inert gas, adverse effects due to oxygen and water or harmful impurity atoms in the air are reduced, preventing formation of oxide films or the like on the surface. However, such a method which employs normal stagger structures is not suitable for application to the liquid crystal display devices as its process increases requiring light-shutter films or the like. In addition, although some improvement has been made in the carrier mobility, as part of the film being changed to p-Si by laser annealing, the interface between a p-Si layer and an a-Si layer cannot be made smooth. Furthermore, the p-Si layer consists of an upper region having a fairly large crystal grain size, a middle region having a small crystal grain size, and a lower region having an irregular boundary. Therefore, when a reverse voltage is applied to the gate, electric field is concentrated on the irregular boundary, giving rise to a problem to cause a large leakage current. Hence, there occurred a problem that peripheral driver circuits could not be integrated in a display substrate. Furthermore, although there were given some considerations against contamination due to inclusion of oxygen atoms, carbon or other impurities into thin films during laser annealing or for preventing formation of oxide films, no effective measures have been taken to assure homogeneity of crystal grains in polycrystalline semiconductor thin films obtained, or with respect to a distribution of crystal grains in a perpendicular direction, thereby giving rise to a problem that the electrical properties became varied.